The painted turtles, Chrysemys picta, with four subspecies, have the largest range and most northern distribution of any North American freshwater turtle. C. p. bellii in Canada may hibernate for as much as 6 mo, while C. p. dorsalis on the Gulf Coast may not hibernate at all. We subjected both subspecies to simulated hibernation conditions by submerging them in anoxic water at 3@?C without access to air to compare physiological responses. Initial acid-base and ionic statuses of animals acclimated to 3@? were the same in both subspecies. As anoxia continued, both subspecies showed similar changes in all measured parameters, but the rates of change were greater in dorsalis than in bellii. By day 46, the plasma pH of dorsalis had fallen to 6.96, a change that required @?180 d in bellii. By day 38, the acidosis was entirely due to lactate formation in both groups, and it had induced compensatory changes in ionic concentrations, particularly large increases in total Ca and total Mg in the blood. We conclude that the only adaptation required of bellii to enable it to endure longer winters than dorsalis is an ability to limit lactate accumulation, by having either a lower metabolic rate and/or a great ability to eliminate lactate. It is known that hibernation in a normoxic environment would be less stressful physiologically than in an anoxic one, but turtles apparently choose the latter. We suggest that the physiological stress of prolonged anoxia and the requirements of subsequent recovery in the spring are outweighed by considerations of predator avoidance during the winter. Tolerance of the physiological perturbations resulting from prolonged anoxia may limit the northern distribution of freshwater turtles.
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